How do I attach a mesh to a rig?
Have you tried the Automatic weight?

Did you do the retopology?

The first time I tried doing Rig, it didn't work until I learned about retopology, wich is (im not that sure) putting less ''points''. I don't know how to explain that correctly, you should watch the video of FlippedNormal on retopology for begginer.

If you already learned about this, maybe look if you have only square, for me when I have triangles it dosen't work.

Hope it will help. Good luck